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stability and performance of low pressure centrifugal compressor provided with diffuser splitter vanes

Omar M. E. Abdel-Hafez, Ahmed S. Hassan and Hany A. Mohamed

Mechanical Engineering Department, Assiut University, Assiut, Egypt.

Influences of installing splitter vanes through the main diffuser vanes at different positions on the performance and the stability of low-pressure compressor were experimentally investigated. Effect of diffuser splitter vane positions on the compressor characteristics; pressure coefficient and efficiency as well as specific power, was studied at different compressor operating conditions. The distributions of the pressure rise at the vaneless region, through the diffuser passages and at diffuser exit were recorded.

In addition to the conventional measurements for compressor characteristics, the time variations of static pressure at diffuser inlet and exit were measured and the waveforms were recorded for different operating conditions under steady or unsteady operations. The recorded pressure waveforms were discussed in relation to rotating stall initiation and surge with the power spectrum density (PSD) using Fast Fourier Transformation analysis (FFT). Based on this analysis, the limiting flow coefficients for stable operations at low flow rates were estimated for diffuser without splitter and with different splitter vane positions. The results indicate that the flow coefficient for stall and surge was decreased with installing splitter vanes through the main diffuser vanes. The present experimental results show acceptable agreement compared to the available literature for another author.

KEYWORDS: Splitter vanes, Centrifugal compressor, Range of stable flow operation, Pressure rise coefficient, Flow coefficient

NOMENCLATURE

b2 Impeller outlet width, m

Pr Pressure ratio

p Static pressure, N/m2

r2 Impeller outer radius, m

R Radius relative to the impeller tip radius

S Splitter length relative to main diffuser vane length

U2 Impeller tip speed, m/s

Greek Symbols

( Density, kg/m3

[pic] Pressure rise, N/m2

[pic] Efficiency,[pic]=[pic]

Φ Flow coefficient = Q/2[pic]b2r2U2

Ψ Pressure coefficient =2[pic]/[pic][pic]

σ Slip factor

Abbreviations

PS Main diffuser van pressure side

SS Main diffuser van suction side

1- INTRODUCTION

It is well known, that the performance of a centrifugal compressor is directly related to the characteristics of the flow within its passages. One of the main concerns of compressors designers and users is to preserve the stability of such flow over a wide range of operating conditions. Numerous studies and investigations have been carried out over the last few decades to identify the characteristics of flow unsteadiness and propose design criteria and practical operational measures to limit their occurrence and expand the stable operating range. The limiting working conditions for stable operation at reduced flow rates has dragged more attention in those studies because, it usually occurs very close to the point of the highest efficiency as mentioned in [1], and is most often acts a trigger of system surge.

Detailed literature survey of flow instability problems in centrifugal compressors diffusers shows that massive number of researches were focused on the inspection of different aspects of flow unsteadiness that lead to the development of stall in centrifugal compressors vaneless diffusers, e.g. [2, 3], and vaned diffusers, e.g. [4]. Most of these researches have been devoted to study the effects of diffuser geometrical parameters and configuration modification on the flow behavior and unsteadiness in both vaneless diffusers, e.g. [5-7], and vaned diffusers, e.g. [1, 8-10]. One of the configuration modification that has been considered in many studies is the installing of splitters or small vanes either in the vaneless space near the diffuser inlet, e.g. [7, 8], or inside the vaned diffuser passages, e.g. [9, 11]. Nakagawa et al. [8] indicated that the performance improvement due to installing small vanes near the diffuser inlet depends highly on the degree of damper opening. It was not possible to investigate the effect of small vanes geometry and position in such experimental investigations as that of [8], because of the complication of the required test rig as well as the tedious procedures needed. Such effects of geometry and positions have been highlighted by Drtina et al. [9] in their numerical study, but for the case of splitters installed within the vaned diffuser passages. They found that the flow is sensitive to the position and geometry of the splitter blades. The effect of installing splitter vanes with different lengths at the middle of the diffuser passage was experimentally investigated in [11] where length of splitter of about 0.1 relative to the main diffuser vane length was found to give the best performance. To the author’s knowledge, these effects of the geometry, number and positions of small vanes installed in the vaned diffuser of a centrifugal compressor have not been fully investigated.

In the present work the effect of installing short splitter vanes (one or two vanes with length of 0.1 relative to the main diffuser vane length) in the diffuser passages on the overall compressor performance were experimentally investigated. In addition, the effects of changing the radial position of those vanes from the diffuser exit toward the diffuser inlet on the stable operation limit at reduced flow conditions as well as on the compressor pressure coefficient are studied. Moreover, the effects of changing the circumferential position from the diffuser main vanes pressure sides to the suction sides were investigated. The Fast Fourier Transformation Analysis (FFT) to estimate the Power Spectrum Density (PSD) is used to study the unsteady flow phenomena and to detect the stall initiation and surge trigger.

2. COMPRESSOR TEST FACILITY

Figure 1 shows the schematic drawing and the photographic picture of the open loop centrifugal compressor test facility used in the experimental work. The compressor is released from an actual aircraft turbocharger (Allis Chalmers type, AN D 132, licensed under general electric company of serial No. 57153 CH, A.F. model 7S-B22-A6, made in USA). A 3.7 kW variable speeds AC motor with speed up to 6000 rpm is coupled with the compressor instead of turbocharger turbine. The motor speed is measured with accuracy of ±10 rpm by using available tachometer. This compressor is constructed from radial blade rotor, parabolic vanes diffuser and volute casing. The compressor draws air at atmospheric conditions and discharges into a large tank followed by an orifice flow meter and control valve for measuring and controlling the amount of the flow rate.

Conventional static pressure and temperature tapes are located through the compressor flow system for securing the overall compressor performance characteristics. Conventional wall pressure taps are installed along the front casing of the whole compressor passage, as shown in Fig.2, to detect the pressure rise between these taps and the inlet to the compressor. Those taps are located in front of the impeller passage (points 1 through 9), in the vaneless space between the impeller exit and diffuser vanes inlet (points 10 through 17) and at diffuser exit (points 18 through 30). Average values of the pressure at the diffuser inlet and exit may be estimated from the readings of the pressure at taps 10 to 17 and 18 to 30 respectively. Short vanes called splitters are designed and located in the middle between the main diffuser vanes, as shown in fig. 3. Experimental investigations were carried out to determine the performance characteristics, for the different cases of the compressor, without splitters and with splitters at different radial and tangential positions.

Three high sensitivity semiconductor-type pressure transducers of omega type, PX-236-100GV silicon diaphragm with full bridge are incorporated jointed to the compressor casing. Two transducers are jointed into the vaneless region with 90o shifted through the circumference at same radius. A DC amplifier (SENSOTEC'S SA-BII) receives the output signals from the pressure transducers and provides a 16-bit A/D converter board (multi-function acquisition card) supported into PC-SCOPE software for simultaneously pressure for one second at a rate of 1 kHz. The board is supported by PC-SCOPE software, which turns the computer to oscilloscope and stores the pressure waveforms in ASCII file. Subsequently the data in the file were processed using the Fast Fourier Transformation Analysis (FFT) to estimate the Power Spectrum Density (PSD) by Welch’s averaged, modified periodogram method for discrete-time signal vector. The arrangements of the pressure signals from the pressure transducers and the recording system, which were used for studying the steady and the unsteady flow phenomena are shown in Fig. 3. The detection of stall initiation and surge trigger is based on examining all the static pressure time variation and PSD graphs at the inlet and the exit of the diffuser of the measured data under different flow rate (about 19 different operating conditions) for the different cases of the compressor, with and without splitters at different positions.

[pic] [pic]

Fig.1 Compressor test facility

[pic]

Fig.2 Locations of pressure taps on the compressor front casing

[pic]

Experimental Schemes:

The experiments works were carried out using the original compressor without any modification in the entire compressor parts, Fig. 4-a, and then the compressor diffuser was modified for next tests as in Fig. 4 as follow:

1- A single short splitter vane (S=0.1) was located at different positions i.e. at middle, Fig. 4-b, near the pressure side, Fig. 4-c, and near the suction side of the diffuser exit (R=1.43), Fig. 4-d, and at the middle in the diffuser passage at different radial positions, R= 1.27 and 1.12.

2- Two short splitters (S=0.1) were located at different positions i.e. parallel with equal apart space at diffuser exit (R=1.43) , Fig. 4-e, longitudinal at middle of the diffuser exit (S=0.2) , Fig. 4-f, staggered with the outer one at the diffuser exit near the suction side, Fig. 4-g, staggered with the outer one at the diffuser exit near the pressure side at the diffuser exit, Fig. 4-h, and parallel with equal apart space near diffuser exit (R=1.27) , Fig. 4-i.

3. EXPERIMENTAL RESULTS AND DISCUSSIONS

3.1 Effect of diffuser splitter vanes on compressor characteristics

Figure 5 shows the performance characteristics of the compressor without splitter in the diffuser (original) and with different splitter vanes installed at different positions between the main diffuser vanes. Figure 5-a shows the effect of installing the splitter at the middle of the main diffuser vanes passage at different radial positions. Figure 5-b shows the effect of installing one splitter vane at the pressure and suction side between the main diffuser vanes at different radial positions. Figure 5-c shows the effect of installing two splitters between the main diffuser vanes passage at different radial positions. It is clear in Figs. 5a-5c that the compressor with one splitter installed between diffuser main vanes near the pressure side (at R=r/r2=1.27) gives the highest pressure coefficient.

[pic]

|c-One splitter, S=0.1, near the|b-One splitter, |a-The original |

|PS of diffuser exit |S=0.1, at |diffuser |

| |middle of diffuser | |

| |exit | |

[pic]

| f-One splitter, S=0.2, |e-Two splitters, |d-One splitter, S=0.1, near|

|at middle of diffuser |S=0.1, at diffuser |the SS at diffuser exit |

|exit |exit | |

[pic]

|i-Two splitters, S=0.1,|h-Two stagger |g-Two stagger splitters,|

|near |splitters, S=0.1, the|S=0.1, the outer at SS|

|diffuser exit |outer at PS | |

Fig.4 One and two splitter vanes at different positions in the main diffuser passages

[pic]

Fig.5-a Compressor characteristics with one splitter

Figure 6 shows the effect of diffuser splitter on the compressor efficiency. It shows that the compressor with one splitter installed between diffuser main vanes near the pressure side, at R=r/r2=1.27 gives the highest efficiency. The other splitters didn’t show any improvements in the compressor efficiency.

[pic]

Fig.5-b Compressor characteristics with one splitter installed at different circumferential positions of the pressure and suction sides of diffuser vanes

[pic]

Fig.5-c Compressor characteristics with two splitters inside diffuser passages

[pic]

Fig. 6 Compressor efficiency with different splitter number and positions

3.2 Effect of diffuser splitter vanes on diffuser inlet pressure fluctuations and power spectrum density

The effects of splitter vanes on flow stability inside the compressor are shown in Figs.7-11. Figure 7 shows the time variation of pressure coefficient and the corresponding power spectra density for the compressor without splitter vanes between the main diffuser vanes (original compressor), at four different operating conditions; Φ = 0.293, 0.204, 0.131 and 0.104. These four flow conditions were selected (from about 20 conditions) to describe the flow during its progressing from steady flow (Φ = 0.293 to more than 0.131), to the initiation of instability (Φ = 0.131) and to surge trigger (Φ = 0.104). It is observed that at the higher flow rate, Φ = 0.293, Fig.7a, the compressor runs stably where both the amplitude of pressure coefficient fluctuations and the corresponding PSD at the diffuser inlet are very small. At the operating points between the maximum flow rate, Φ = 0.293 to more than Φ = 0.131, Fig.7a-7b, the compressor show small amplitudes of pressure fluctuations and PSD where the compressor is still operating in the range of stable operation. When the compressor was operating at the flow coefficient Φ = 0.131, as shown in Fig.7-c, transient pressure measurements at the diffuser inlet shows that the amplitude of pressure fluctuations reaches about 38% of compressor pressure coefficient at this flow condition with frequency of 32 Hz, where the compressor is expected to begin to run in present of rotating stall. As the compressor operates at more low flow rate, Φ = 0.104, the amplitude of pressure fluctuations further increases. At this operating point, the values of fluctuations of the pressure coefficient are about 45% of compressor pressure coefficient, as shown in Fig.7d and the frequency of these pressure fluctuations has different predominant frequencies of 4 Hz and 13-37 Hz. Since, the amplitude of pressure fluctuation is so high with low frequency of 4 Hz, which lies in the range of surge frequency (1-10 Hz), then the present flow situation can be described as triggering of surge.

a- φ =0.293 [pic][pic]

b− φ ’ 0.204 [pic][pic]

c- φ = 0.131

[pic][pic]

e- φ = 0.104

[pic][pic]

Fig.7 Time variation and power spectrum density of static pressure for compressor without diffuser splitter

Figure 8 shows the time variation of pressure coefficient for the compressor of a diffuser has two splitter vanes at the diffuser exit, one near the pressure side and one near the suction side of the main diffuser vanes at R=1.43. This figure shows the at compressor maximum flow rate the amplitude of fluctuation in pressure coefficient is about 20% of compressor maximum pressure coefficient with different frequencies and the amplitudes of the power spectrum density (PSD) at the low frequencies are relatively low but this is unsteady flow phenomena due to installing the two splitters at the exit of the diffuser. With decreasing the flow rate to Φ = 0.28 the flow becomes more complicated which appears from the higher amplitude of PSD with low frequencies. This means that the splitter at this position causes more separation and blockage at the diffuser exit and this is confirmed with the decrease in pressure coefficient as shown in the compressor characteristic of Fig.5-c.

When the two splitters were located as staggered splitters, one is located near the suction side of the main diffuser vane at R = 1.43 and the other is located near the pressure side at R = 1.27, Fig.4-g, the compressor runs stably even at very small flow rate conditions, e.g. at Φ = 0.036, as shown in Fig. 9-d. At this flow rate the amplitude of pressure fluctuations is relatively high but the PSD is very small relative to the above mentioned values for the case of the two splitters at the diffuser exit. This is due to operates the compressor in stable operation without flow separation. This figure clearly shows that at very low flow rate, Figures 10 and 11 show the time variations of static pressure coefficients and PSD for the compressor with splitter at the middle (at R=1.43) and at the pressure side (at R=1.27) respectively. Installing the splitter at the two above-mentioned cases increases the flow stability than the compressor without splitter vanes.

a- φ =0.282 [pic][pic]

b-φ = 0.248 [pic][pic]

c-φ = 0.184 [pic][pic]d- Φ = 0.15 [pic][pic]

Fig.8 Two splitters one near suction side and one near pressure side at R=1.43

a-φ=0.29

[pic][pic]

b-φ=0.183

[pic][pic]

c-φ= 0.138

[pic][pic]

d-φ=0.088

[pic][pic]

e-φ=0.036 [pic][pic]

Fig.9 Time variation and power spectrum density of static pressure for the compressor with two stagger splitters, one at suction side at R=1.43 and the other at pressure side at R =1.27

a-φ=0.29

[pic][pic]

b-φ=0.148

[pic][pic]

c-φ=0.148

[pic][pic]

d-φ=0.094

[pic] [pic]

e-φ=0.0356

[pic][pic]

Fig.10 Time variation and power spectrum density of static pressure for the compressor with one splitter installed at the middle of diffuser channel at R=1.43

a-φ=0.294

[pic][pic]

b-φ=0.096

[pic][pic]

c-φ=0.082 [pic][pic]

d-φ=0.066

[pic][pic]

Fig.11 Time variation and power spectrum density of static pressure for the compressor with one splitter at the diffuser pressure side, R=1.27

4. CONCLUSIONS

Effect of installing of short splitter vanes between the main diffuser vanes on the performance characteristics of the low-pressure compressor has been experimentally investigated. The positions of the short splitter vanes (one or two short vanes with length of 0.1 relative to the main diffuser vane length) were changed radially and relative to each other from the diffuser exit toward the diffuser inlet. In addition, the positions of these short vanes were changed circumferentially from the diffuser main vanes pressure sides to the suction sides of them.

The fluctuations of pressure in time domains were measured at diffuser inlet for different steady and flow operating conditions. The recorded pressure waveforms were discussed based on rotating stall initiation and surge using the power spectrum density (PSD) and Fast Fourier Transformation analysis (FFT). According to these measurements, the mass flow rate at which rotating stall was initiated and the limits of stable operation were estimated with different splitters. The results of testing the new diffusers that fitted with short vanes splitters are compared with the original diffuser (diffuser without splitter). It is concluded from these results that the new diffuser that having short vanes (S=0.1) between the diffuser main vanes at position near pressure side of main diffuser vanes (at R=1.27) gives higher-pressure coefficient and increases the range of stable operation which is free from rotating stall. The diffuser with two stagger splitters, one at suction side at R=1.43 and the other at pressure side at R =1.27, gives an increase in stable operating range in comparison with the diffuser without splitter.

REFERENCES

1. Ogata, M. and Ariga, I., “An Experimental Study of Rotating Stall in Radial Vaned Diffuser”, Proceedings of 7th International Symposium on Unsteady Aerodynamic and Aero elasticity of Turbomachines (ISUAAT), Fukuoka, Japan, pp. 625-639, 1994.

2. Inoue, M. and Cumpst, N. A., “Experimental Study of Centrifugal Impeller Discharge Flow in Vaneless Vaned Diffusers”, J. of Engineering for Gas Turbines and Power, Vol. 106, pp. 455-467, April 1984.

3. Shin, Y.H., Kim, K.H. and Son, B. J., “An Experimental Study on the Development of a Reverse Flow Zone in a Vaneless Diffuser”, JSME, Series B, Vol. 44, No. 3, pp. 546-555, 1998.

4. Dawes, W.N. “A Simulation of Unsteady Interaction of Centrifugal Impeller with its Vaned Diffuser: Flow Analysis”, J. of Turbomachinery, Vol. 117, pp.213-222. 1995.

5. Ishida, M., Sakaguchi, D., and Ueki, H. “Detection of Rotating Stall Precursor in Vaneless Diffuser of a Centrifugal Blower”, ASME paper, FED-Vol. 222, Fluid Machinery, 1995.

6. Frigne, P. and Van Den Braembussche, R.,” Distinction Between Different Types of Impeller and Diffuser Rotating Stall in a Centrifugal Compressor with Vaneless Diffuser, Trans. ASME, Vol. 106, pp. 468-474, 1984.

7. Konishi, T., Sakai, T., and Whitfield, A., “Performance Improvement of a Mixed-Flow Fan Through the Application of Guide Fences in the Vaneless Diffuser”, Proc. Institute of Mechanical Engineering, Vol. 212, Part A, pp. 217-2224. 1998.

8. Nakagawa, K., Keimi, Y., and Nishioka, T., “Improved Flow Range in Radial Compressor with Vaned Diffuser”, ASME paper, FED-Vol. 222, Fluid Machinery. 1995.

9. Drtina, P., Dalbert, P., Rutti, K., and Schachenmann, A., “Optimization of a Diffuser with Splitter by Numerical Simulation”, ASME paper 93-GT-110, 1993.

10. Eynon, P.A., and Whitfield, A., “The Effect of Low-Solidity Vaned Diffusers on the Performance of a Turbocharger Compressor”, Proc. of the Institute of Mech. Engineering, Vol. 211. 1997.

11. Abdel-Hafez,O.M.E., “Influence of Diffuser Splitter Vans on the Performance of Low Pressure Centrifugal Compressor”, Journal of Engineering Sciences, Faculty of Engineering-University of Assiut, Egypt, Vol. 32-No. 1, Part A, January 2004.

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